In recent years, thermal transfer systems have been developed to obtain prints from pictures that have been generated from a camera or scanning device. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye donor element is placed face-to-face with a thermal image receiver element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated sequentially in response to one of the cyan, magenta or yellow signals. The process is then repeated for the other colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen.
Various approaches have been suggested for providing a thermal dye receiving layer. Solvent coating of the dye image receiving layer formulation is a common approach. However, the use of solvents to coat these formulations brings with it various problems including expense, environmental hazards and waste concerns, and hazardous manufacturing processes. Special precautions are required to manage these problems. For example, organic solvent coated formulations and methods are described in U.S. Pat. No. 5,356,859 (Lum et al.).
Another approach involves hot-melt extrusion of the dye image receiving layer formulation onto a support. Multiple layers can be co-extruded in the preparation of the thermal image receiver element. Such methods are highly effective to prepare useful thermal image receiver elements but they restrict the type of materials that can be incorporated into the dye image receiving layer due to the high temperatures used for the extrusion process. U.S. Pat. No. 7,993,559 (Dontula et al.) and U.S. Patent Application Publication 2010/0330306 (Dontula et al.) describe imaging elements having multiple extruded layers included extruded compliant and antistatic subbing layers. U.S. Patent Application Publication 2008/0220190 (Majumdar et al.) describes image recording elements comprising a support having thereon an aqueous subbing layer and an extruded dye receiving layer. In addition, U.S. Patent Application Publications 2011/0091667 (Majumdar et al.) and 2010/0330306 (Dontula et al.) describe thermal dye transfer receiver elements that include an extruded compliant layer and an antistatic layer adhering it to an image receiving layer.
Yet another approach is to use aqueous coating formulations to prepare the dye image receiving layers. Such formulations typically include a water-soluble or water-dispersible polymer as the binder matrix. Some efforts to do make such formulations are described for example U.S. Patent Application Publications 2011/0027505 (Majumdar et al.) and 2011/0117299 (Kung et al.).
Although aqueous coating methods and formulations are desired for the noted reasons, aqueous-coated dye image receiving layers can exhibit problems in typical customer printing environments where high speed printing requires a smooth separation of dye donor element and the thermal image receiver element with no sticking between the contacting surfaces of the two elements. Printing such images in high humidity environments can be particularly troublesome for sticking with aqueous-coated dye image receiver layers. Moreover, such thermal image receiver elements are often deficient in providing adequate dye density in the thermally formed images. Aqueous-coated layers can also fall apart when contacted with water.
The industry has aggressively approached these problems with various proposed solutions that are described in the literature. For example, U.S. Patent Application Publication 2009/0061124 (Koide et al.) describes the use of various latex polymers in dye image receiving layers, which latex polymers are generally prepared at least in part from vinyl chloride. Alternatively, U.S. Pat. No. 7,820,359 (Yoshitani et al.) describes the use of latex polymers in dye image receiving layers, which latex polymers are derived from specific monomers having alkyleneoxy side chains and either an unsaturated nitrile, styrene, or styrene derivative.
Despite all of the known approaches to the various problems associated with the use of aqueous coated dye image receiving layer formulations, there continues to be a need to improve the resistance of such formulations (and the dried layers obtained therefrom) to changes in relative humidity so that the resulting images are consistent and exhibit sufficient density, no matter the relative humidity in which the thermal dye transfer elements are stored or used.